When TESSDATA_PREFIX points at a hand-assembled tessdata folder lacking
the configs/ subdirectory (e.g. files pulled from tessdata_best),
Tesseract prints "read_params_file: Can't open hocr/txt" and produces no
output. The runtime now surfaces a clear error (v17.5.0); add the
matching documentation: a new errors.md entry and a note on the
TESSDATA_PREFIX docs.